core: cleanup Handler classes

Change the visibility of ObjectIdentityChecker instance in VmHandler,
VdsHander and VmTemplateHandler to private and replace direct access to
those fields from outside of those classes, with delegation methods calls

Change-Id: I9dbea95ff90f0fc638c2f109578e932bc964e8f9
Signed-off-by: Arik Hadas <ahadas@redhat.com>